The petrology of a potassic syenite and its associated biotite pyroxenite at Shaki, Western Nigeria

Abstract

A syenite gneiss associated with biotite pyroxenite and biotite-muscovite gneiss forms an elongated mass covering about 150 km² in the basement complex around Shaki in Western Nigeria. It lies conformably in the biotite-museovite gneiss to which it is similar in texture. The biotite pyroxenite occurs as patches of varying sizes widely distributed throughout the syenite but is not found in any of the surrounding rocks.

18 chemical analyses and 62 modal analyses show that the syenite is composed essentially of microcline (in places slightly perthitic), albite, quartz, diopsidic augite and hornblende and is chemically characterised by the unusual combination of very high K₂O with high MgO, FeO and CaO contents. The biotite pyroxenite and the syenite contain high amounts of the trace elements characteristic of both magnesian ultra-basic rocks and granitic rocks.

The syenite and the biotite pyroxenite are believed to have originated through two metasomatic alteration processes; one characterised by CaO+FeO+MgO and the other by K₂O. The two processes are believed to have been simultaneous and related. The process may be analogous to that obtaining during fenitisation.